1. Field of the Invention
The present invention relates to silicon carbide semiconductor devices and methods for manufacturing the same, and more particularly to a silicon carbide semiconductor device capable of suppressing threshold voltage variation and a method for manufacturing the same.
2. Description of the Background Art
In recent years, silicon carbide has been increasingly employed as a material for a semiconductor device in order to allow a higher breakdown voltage, lower loss and the use in a high-temperature environment and the like of the semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap wider than that of silicon which has been conventionally and widely used as a material for a semiconductor device. By employing the silicon carbide as a material for a semiconductor device, therefore, a higher breakdown voltage, lower on-resistance and the like of the semiconductor device can be achieved. A semiconductor device made of silicon carbide also has the advantage of exhibiting less performance degradation when used in a high-temperature environment than a semiconductor device made of silicon.
Among the semiconductor devices made of silicon carbide, in a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an IGBT (Insulated Gate Bipolar Transistor), for example, an electric current can be passed and interrupted between two electrodes by controlling the presence or absence of formation of an inversion layer in a channel region based on a prescribed threshold voltage.
For example, Mitsuo Okamoto et al., “Reduction of instability in Vth of 4H—SiC C-face MOSFETs,” Proceedings of the 59th Spring Meeting of The Japan Society of Applied Physics and Related Societies (2012 Waseda University), 15-309 (Non-Patent Document 1) points out that a threshold voltage varies with gate bias stress in a silicon carbide MOSFET. In order to reduce variation in the threshold voltage, Non-Patent Document 1 discloses a method of annealing a silicon carbide substrate having a gate oxide film formed thereon in a hydrogen atmosphere.
However, if annealing is performed in a hydrogen atmosphere, even if the threshold voltage variation can be temporarily reduced, it is believed that the effect of the reduction in threshold voltage variation is lost when the substrate is exposed to high temperature in a subsequent step of forming an ohmic electrode, for example. In other words, although the threshold voltage variation is reduced at a stage where a gate electrode is formed on a substrate, it is believed that the threshold voltage variation is not reduced at a stage where a final device is produced.